Fatigue Testing of Used Mooring Chain

Abstract

In Statoil mooring chains have been inspected since the first floaters were installed in the 1990’s, and replacement of top chain started as early as year 2000. Background for replacement has been wear, surface corrosion, end of design fatigue life and check of condition for lifetime extension evaluation. For seabed chains in site inspection has been, and still are, difficult and expensive. High quality inspection data is limited. Technology development the last years has allowed for cost efficient seabed chain replacements, and presently even seabed chains have been retrieved to investigate integrity and remaining fatigue life. Statoil started full scale fatigue testing of chains in 2011, and the results from the first tests, presented in Fredheim et al (2013), indicates that the surface condition of the chain is vital for the fatigue capacity. Fatigue test show that chain fatigue capacity varies significantly. The surface conditions of the chains are site dependent. To ensure timely replacement, it is vital to know the remaining fatigue capacity of the chains. An extensive fatigue test program was thus initiated, and is still ongoing. As testing has proceeded, different degrading mechanisms and fatigue capacity dependencies have been revealed. Further testing is still required, and more tests are scheduled for the following years. The paper sums up testing so far (Dec 2016), and presents the most important findings and results.